1. Field of the Invention
The invention relates to a process for producing a metal foam in a metal die-casting machine, in particular for a producing metal foam from aluminum materials. This invention further provides for a metal body produced using this process; for example, a component made from an aluminum material of this type.
2. Discussion of the Related Art
It is known to produce components from foamed metallic materials. These components are distinguished by inter alia a lightweight structure, rigidity, compressive strength improved mechanical and acoustic damping. By way of example, cast cores of aluminum foam are surrounded with an aluminum material by casting or are inserted as shaped parts into a component The sheath and core or shaped part are produced separately and are then joined to one another. In addition to the high manufacturing outlay, this also leads to a low manufacturing quality.
The basis of foamable semifinished aluminum products is atomized aluminum powder to which a blowing agent is added. By way of example, according to DE-A-197 44 300, a body which has been pressed from a powder mixture is heated, in a heatable, closed vessel, to temperatures which are higher than the decomposition temperature of the blowing agent and/or the melting temperature of the metal.
The powder is compressed and the shaped part produced in this way is inserted into the area of a component, which is to be foamed. The powder is foamed by being heated to up to 650° C. In the process, the “sheath” may be subject to unacceptable deformation or the foaming operation may take place non-uniformly.
Sintering of metallic hollow beads or infiltration of metal melts into cores or filler bodies, which are removed after solidification of the melt, is also possible.
According to a process described in JP-A-03017236, metallic articles with cavities are produced by dissolving gases in a metal melt and initiating the foaming operation by suddenly reducing the pressure. The foam is stabilized by cooling of the melt.
According to the teaching given in JP-A-09241780, metallic foam is obtained with the controlled release of blowing gases as a result of a metal initially being melted at temperatures which lie below the decomposition temperature of the blowing agent used. Subsequent dispersion of the blowing agent in the molten metal and heating of the matrix to above the temperature which is then required to release blowing gases leads to a metal foam being formed.
W. Thiele: Füllstoffhaltiger Alumumschwamm—ein kompressibler Gusswerkstoff zur Absorption von Stoβenergie, [Filler-containing aluminum sponge—a compressible cast material for absorption of impact energy], in: Metall, 28, 1974, Vol. 1, pp. 39 to 42, describes the production of foamed aluminum. The desired cavities are predetermined in terms of size, shape and position in the form of a loose bed of readily compressible, inorganic light materials, such as for example expanded clay minerals, expanded clay, glass foam beads or hollow corundum beads, etc. The bed of light material is introduced into a die. The spaces which remain in the bed are filled with metal. The aluminum sponge obtained in this way has relatively poor mechanical qualities and contains the materials of the bed.
DE-B-11 64 103 describes a process for producing metal foam bodies. In this process, a solid material which, when heated, decomposes to form gases, is mixed with a molten metal in such a manner that the solid material is wetted by the metal. By way of example, pulverulent titanium hydride is added to a molten alloy of aluminum and magnesium at a temperature of 600° C. The closed foam formed in this way is then cast into a die, where it can cool and solidify. In this case too, it is clearly not a closed system, but rather an open system which is used.
GB-A 892934 describes the production of complex structures with foamed metal core and closed, nonporous surface, the execution of the described process being dependent on the introduction of the metal/blowing agent mixture into the die before the start of the foaming process.
DE-C 198 32 794 describes a process for producing a hollow profiled section which is filled with metal foam. This process comprises the steps of extruding the hollow profiled section from a sheathing material using an extruder that has an extrusion die with a die part and a mandrel, supplying the metal foam from a foam material to the hollow profiled section through a feed duct, which is formed in the mandrel.
The casting of metal parts with lost foam is already known in accordance with EP-B 0 461 052. WO 92/21457 describes the production of aluminum foam in such a manner that gas is blown in under the surface of a molten metal with abrasives being used as stabilizers.
EP-B 0 666 784 describes a process for the shape casting of a metal foam which is stabilized by means of particles, in particular an aluminum alloy, by heating a composite of a metal matrix and finely divided solid stabilizer particles above the solidus temperature of the metal matrix and discharging gas bubbles into the molten metal composite below the surface thereof to form a stabilized liquid foam on the surface of the molten metal composite. The characterizing feature is shape casting of the metal foam by the stabilized, liquid foam being pressed into a mold, using a pressure which is just sufficient for the liquid foam to adopt the shape of the mold, without the cells of the foam being significantly compressed, and then cooling and solidifying the foam, in order to obtain a shaped object. The foam is in this case pressed into the mold by means of a moveable plate. A first moveable plate presses the liquid foam into the mold and a smooth surface is formed on the shaped foam object. In order to form smooth inner surfaces on the foam object, a second moveable plate is pressed into the foam inside the mold. However, the shaping may also take place by means of rollers.
A further process for making castings from metal foam is described in EP-A 0 804 982. In this case, the foaming takes place in a heatable chamber outside a casting mold, the volume of the powder metallurgy starting material introduced into the chamber for the metal foam, in its phase in which it has been foamed with the entire foaming capacity, substantially corresponding to the volume of a filled mold. All the metal foam in the chamber is pressed into the mold, in which foaming with the remaining foaming capacity is continued until the mold has been completely filled. The mold is a sand or ceramic mold, the metal foam is inserted into the chamber as a semifinished product and is only pressed into the mold, for example by means of a piston, after the initial foaming. When the foam is being pressed into the mold, it is sheared. The mold is not filled with a foam with a structure which is deliberately inhomogeneous.
DE-A 19 501 508 discloses a process for producing a hollow profiled section of reduced weight and increased rigidity, for example a component for the chassis of a motor vehicle. It comprises die-cast aluminum, in the cavities of which there is a core of aluminum foam. The integrated foam core is produced by powder metallurgy and is then fixed to the inner wall of a casting die and surrounded with metal by die-casting.
The dissolving or blowing of blowing gases into metal melts is not suitable for the production of near net shape components, since a system comprising melt with occluded gas bubbles is not stable for a sufficient time for it to be processed in shaping dies.